A great deal is known concerning the physiology, pharmacology and efferent anatomy of the noradrenergic locus coeruleus (LC) system; in contrast, current knowledge of afferent regulation of this important brain system is markedly deficient. In fact, our recent results indicate that most previous conclusions concerning inputs to LC were erroneous: the vast majority of afferents to LC arise from only two loci, the nuclei paragigantocellularis (PGi) and prepositus hypoglossi (PrH), both in rostral medulla. The long term goal of the proposed research is to elucidate the anatomical, physiological and neurochemical mechanisms that control LC activity; the present cellular integrative studies of these newly discovered afferents to LC are the first steps in achieving this end. Inputs to LC from its major afferents, PGi and PrH, will be characterized using anterogradely transported phaseolus vulgaris- leuooagglutinin (PHA-L). Fiber lengths, branching, densities an swellings throughout LC will be compared for transport from subfields of PGi and PrH. In addition, possible afferents to LC from adjacent nuclei, difficult to assess in our previous WGA-HRP studies, will be examined using PHA-L anterograde transport. In a second project, the neurochemical identity of PGi and PrH neurons afferent to LC will be investigated in a series of fluorescence double labeling anatomic studies. Neurons afferent to LC will be identified with a retrograde tracer, and then also characterized for neurotransmitter markers by staining the same material with immunohistochemical probes. In the third project, circuit relationships of the major afferents to LC will be investigated using injections of WGA-HRP into subfields of PGi and PrH. Areas containing retrogradely labeled neurons will then be injected with PHA-L to determine the patterns of fiber innervation in PGi and PrH from their afferents. We will also investigate the roles PGi and PrH play in control of LC discharge by measuring the affects of selectively lesioning these areas of spontaneous LC discharge as well as that evoked by a natural sensory stimulus. Thus, the proposed studies build upon one another, form a direct extension of our recent anatomic results defining sources for inputs to LC, to experiments in which the overall roles of these major afferents in controlling LC activity are examined.